Fine-particle size silica (SiO.sub.2) is a commercialy available and useful product. Fumed, amorphous silica is known to be prepared, e.g., by pyrolyzing silicon halides in processes called vapor phase hydrolysis. When the silicon halide is a silicon chloride, e.g., SiCl.sub.4, the fumed silica formed is hydrophilic. When the silicon halide is a silicon fluoride, e.g., SiF.sub.4 or H.sub.2 SiF.sub.6, the fumed silica formed is hydrophobic. Such hydrophobic silica will lose its hydrophobicity and become hydrophilic when in contact with water for extended periods of time.
Among the commercially available fine-particle silicas are those, e.g., bearing the tradenames Syliod, Cab-O-Sil, Zeosyl, Imsil, and Aerosil.
There is a need for methods of converting hydrophilic silica to hydrophobic silica, even silica which had previously been hydrophobic but which has become hydrophilic.
Patents which teach the preparation of fumed silica are, e.g., U.S. Pat. Nos. 3,053,627; 2,819,151; and 3,203,759.
U.S. Pat. No. 2,535, 036, e.g., teaches to react SiO.sub.2 with HF to form fluorosilicic acid (H.sub.2 SiF.sub.6).
U.S. Pat. No. 3,862,307 teaches to treat SiO.sub.2 with aqueous HF to acidify the SiO.sub.2 and provide fluoride values in dentifrice formulations. It is obvious that when aqueous HF is employed, the SiO.sub.2 is prevented from becoming hydrophobic, since the silica is thoroughly wetted throughout the treating step.
It is an object of the present invention to convert hydrophilic silica to hydrophobic silica.
It is another object to return a previously hydrophobic silica to its hydrophobic state after it has lost its hydrophobicity by having been treated with water or with an agent which has neutralized its fluoride values.
Another object is to improve the oleophilic properties of silica by rendering the silica hydrophobic.
These and other objects, which will become apparent to practitioners of the relevant arts, are attained by the present invention.